As disclosed in U.S. Pat. No. 5,926,079, a plurality of resonators formed in a dielectric block can be coupled together to provide various waveguide-type dielectric filters. FIG. 7 is a perspective view showing one example of such waveguide-type dielectric filters. The dielectric filter comprises a rectangular parallelepiped-shaped dielectric block 70, input and output electrodes 77 provided, respectively, at opposite ends of the dielectric block 70, plural pairs of grooves or slots 79 each disposed between adjacent resonators to extend inward from both the side surfaces of the dielectric block 70 so as to form a coupling iris, and a conductive film covering over the surface of the dielectric block 70 with the slots 79. Each of the coupling irises acts to adjust the coupling between the adjacent resonators. In particular, for obtaining a narrow-band filter, each of the pair of slots is required to have a cutting depth greater than a given value. The increased cutting depth inevitably narrows the width of the coupling iris, which causes deterioration in the strength of the portion of the dielectric filter where the coupling iris is formed.
While a through-hole may be provided between adjacent resonators as a substitute for the slot, this structure involves problems of increase in the process time for forming the through-hole in a dielectric block and difficulty in assuring the working accuracy of the through-hole.